A man from Northern Ireland who abused at least 70 children online and drove one to suicide was sentenced to life in jail with a minimum term of 20 years in a “horrific” case that has caused “catastrophic damage” to young girls all over the world.

Alexander McCartney, 26, had admitted to 185 charges including blackmail, inciting a child to engage in sexual activity and producing and distributing indecent images of children.

McCartney was told he would spend at least 20 years in jail after a number of other sentences which he will serve concurrently were taken into account.

In one of the biggest investigations into “catfishing” or sexual extortion in the world, police found victims aged between 10 and 16 in the UK, Europe, Australia and New Zealand. Prosecutors said he had targeted around 3,500 children.

McCartney, who posed as a young girl to befriend other girls on Snapchat before blackmailing them, is believed to be the UK’s most prolific catfish offender.

Sitting at Belfast crown court, the judge, Justice O’Hara, said: “In my judgment, it is truly difficult to think of a sexual deviant who poses a greater risk than this defendant.

“There has not been a case such as the present where the defendant has used social media on an industrial scale to inflict such terrible and catastrophic damage on young girls, up to and including the death of a 12-year-old girl. The defendant was remorseless. He ignored multiple opportunities to stop he ignored multiple pleas for mercy. He lied and lied and then lied again.”

McCartney was sentenced to life in prison on the counts of manslaughter and on the counts of causing girls under 13 to engage in sexual activity, including penetration.

He was given a number of other sentences ranging from 10 years “in each count” for 45 counts of various degrees of seriousness that caused sexual activity in underage girls, six years in each count for 29 counts of possessing indecent images, and 10 years in each count for 58 counts of blackmail.

Before handing down the sentence, the judge told families of the victims he would spare them the repetition of the “sadism” and the “harrowing nature” of some of the details of McCartney’s crimes.

“This is a quite horrific case. Submissions were made on [McCartney’s] behalf about various mitigating factors … Those mitigating factors are few in number and limited in nature,” O’Hara said.

Sentencing was scheduled at the unusual time of 2pm to allow victims’ families in the US and elsewhere to attend the hearing virtually.

The impact of McCartney’s “depravity” and “sadism” on his victims included “depression, anxiety, stress, shame, embarrassment, loss of confidence, difficulty in trusting others”, said O’Hara.

“For many of them, their childhoods have been stolen. Some have attempted to commit suicide. Others report self-harm and suicidal thoughts.

“Whatever remorse the defendant now seeks to persuade me of, he was absolutely empty of remorse at the time,” he added.

One of his victims, 12-year-old Cimarron Thomas, from West Virginia in the US, killed herself in May 2018 after McCartney threatened to share intimate images with her father.

After retrieving electronic devices from McCartney’s home, investigators discovered three years later that the girl had died by suicide three minutes after the blackmail conversation.

The court heard that Cimarron had become deeply distressed and told McCartney she would go to the police and take her own life. But he said he did not care and gave her a countdown for when he would share the pictures.

Her body was found by her nine-year-old sister and 18 months later her father, Ben Thomas, a US army veteran, also died by suicide, never knowing what led his daughter to take her own life.

In an impact statement, her grandparents said: “Our lives will never be the same. We didn’t get to see her graduate, walk down the aisle or have children. We have been robbed of those memories. Our lives have changed for ever.”

In another exchange, McCartney told a girl he would send people to her home to rape her if she did not comply with his sextortion demands.

O’Hara told of McCartney’s callous and cold response to several victims who told him they would kill themselves or harm themselves. To one, he said “good luck and goodbye”. To another who had said her mother had cancer, he said: “I don’t give a shit.”

As time went on “there was an escalation in the seriousness” and “depravity” of his conduct, O’Hara said.

Operating out of the bedroom in his family home outside Newry, the computer science student posed as a young girl on Snapchat, befriending girls who were gay or exploring their sexuality.

The judge told the court: “His choice of victim was particularly calculating and sinister because the fact that they were exploring homosexuality added an additional layer of security to his actions.”

Once McCartney had secured a picture from his victims, he would then reveal the “catfish” and blackmail them into taking part in sex acts. In some instances, he demanded his victims involve younger siblings, including victims of between three and five years old, O’Hara told the court.

McCartney was arrested several times between 2016 and 2019 but continued to offend despite bail conditions until he was remanded in custody.

At a pre-sentence hearing last week, a prosecuting barrister said McCartney had degraded and humiliated his victims, stating that the harm caused to them was “unquantifiable”.

When McCartney first came to the police’s attention in 2016, they found eight computer towers, four laptops, eight tablets and nine mobile phones. Indecent images of children were found on four other devices.

Catherine Kierans, the acting head of the serious crime unit at the Public Prosecution Service in Northern Ireland said the case was “one of the most distressing and prolific cases of child sexual abuse” they had ever seen.

She said he had targeted 3,500 children, many of whom had sought help on social media.

Some children pleaded for him to stop the abuse but he continued, at times forcing the victims to involve younger children, some aged as young as four.

• In the UK and Ireland, Samaritans can be contacted on freephone 116 123, or email jo@samaritans.org or jo@samaritans.ie. In the US, you can call or text the National Suicide Prevention Lifeline on 988, chat on 988lifeline.org, or text HOME to 741741 to connect with a crisis counselor. In Australia, the crisis support service Lifeline is 13 11 14. Other international helplines can be found at befrienders.org